Only apply the default for app_src of erlang_app_sources if unset #1151

Job	Run time
test (25.3)	2m 34s
test (25.3)	2m 34s
test (26.2)	2m 26s
test (26.2)	2m 26s
test-bzlmod-internal-erlang	17s
test-bzlmod-internal-erlang	17s
test-bzlmod-macos	2m 37s
test-bzlmod-macos	2m 37s
test-bzlmod (25.3)	2m 39s
test-bzlmod (25.3)	2m 39s
test-bzlmod-windows (25.3)	4m 6s
test-bzlmod-windows (25.3)	4m 6s
test-bzlmod (26.2)	2m 36s
test-bzlmod (26.2)	2m 33s
unit-test-bzlmod (25.3)	24s
unit-test-bzlmod (25.3)	24s
unit-test (25.3)	28s
unit-test (25.3)	28s
unit-test-bzlmod (26.2)	23s
unit-test-bzlmod (26.2)	23s
unit-test (26.2)	24s
unit-test (26.2)	24s
test-bzlmod-coverage	2m 34s
test-bzlmod-coverage	2m 34s
test-host-erlang-change-detected	24s
test-host-erlang-change-detected	24s
summary	0s
summary	1s
	43m 42s

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